An inkjet printing system typically includes one or more printheads and their corresponding ink supplies. Each printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors, each ejector including an ink pressurization chamber, an ejecting actuator and an orifice through which droplets of ink are ejected. The ejecting actuator can be one of various types, including a heater that vaporizes some of the ink in the pressurization chamber in order to propel a droplet out of the orifice, or a piezoelectric device which changes the wall geometry of the chamber in order to produce a pressure wave that ejects a droplet. The droplets are typically directed toward paper or other recording medium (sometimes generically referred to as paper herein) in order to produce an image according to image data that is converted into electronic firing pulses for the drop ejectors as the print medium is moved relative to the printhead.
Motion of the print medium relative to the printhead may consist of keeping the printhead stationary and advancing the print medium past the printhead while the drops are ejected. This architecture is appropriate if the nozzle array on the printhead can address the entire region of interest across the width of the print medium. Such printheads are sometimes called pagewidth printheads.
A second type of printer architecture is the carriage printer, where the printhead nozzle array is somewhat smaller than the extent of the region of interest for printing on the print medium and the printhead is mounted on a carriage. In a carriage printer, the print medium is advanced a given distance along a print medium advance direction and then stopped. While the print medium is stopped, the printhead carriage is moved in a direction that is substantially perpendicular to the print medium advance direction as the drops are ejected from the nozzles. After the carriage has printed a swath of the image while traversing the print medium, the print medium is advanced; the carriage direction of motion is reversed; and the image is formed swath by swath.
Inkjet ink includes a variety of volatile and nonvolatile components including pigments or dyes, humectants, image durability enhancers, and carriers or solvents. A key consideration in ink formulation is the ability to produce high quality images on the print medium. During periods when ink is not being ejected from an ejector, the ink viscosity at the nozzle can change. For example, the volatile components of the ink can evaporate through the nozzle. Such changes can make the drop ejection process nonuniform, so that the image quality can be degraded. In addition, dust, dried ink or other particulates can partially block a nozzle or make the wettability of the nozzle face around the nozzle nonuniform so that ejected drops can be misdirected from their intended flight paths.
In order to maintain the drop ejecting quality of the printhead so that high quality images are produced even after periods where one or more nozzles has been inactive, a variety of maintenance actions have been developed and are well known in the art. These maintenance actions can include capping the printhead nozzle face region during periods of nonprinting, wiping the nozzle face, periodically spitting drops from the nozzles into the cap or other reservoir that is outside the printing region, priming the nozzles by applying a suction pressure at the nozzle face.
The extent to which the nozzles of a printhead require maintenance depends upon the environmental conditions (such as humidity and temperature) in the printer, as well as the length of time during which ink has not been ejected. U.S. Pat. No. 5,995,067 discloses providing a humidity sensor as well as a temperature sensor within the printer. Depending upon measured humidity and temperature conditions within the printer, as well as elapsed time, the maintenance is controllably adjusted. For example, for low relative humidity and low temperature, a priming operation is performed. For various combinations of higher humidity and temperature, priming is not required, but various amounts of spitting can be done. For example, for higher levels of humidity, less spitting is required than at lower levels of humidity.
Temperature sensors are provided in many printers, but humidity sensors are found in fewer printers. Jetted ink drop size depends upon temperature for a given set of drop ejection conditions. Excellent and repeatable print quality typically depends upon sensing the temperature and modifying the drop ejection conditions (such as ejection pulse voltage or pulse width or waveform, or number of pulses) to keep the drop size approximately constant. Humidity has a less direct impact upon print quality so many printers do not include a humidity sensor in order to save expense. Humidity information is not available to such printers and maintenance routines are based simply on elapsed time and optionally also on temperature. In order for the maintenance routine to provide satisfactory printing results for all humidity levels, it is typically assumed that the humidity is at a low level. This is effective for providing quality printing, but is wasteful of both ink and time at higher levels of humidity where a less aggressive maintenance routine would suffice.
What is needed is a way to provide humidity information to adjust maintenance routines for printers that do not include a humidity sensor. For most users such humidity information will permit more efficient ink usage and less time spent on maintenance. More efficient ink usage makes it possible for the user to change ink supplies less frequently, saving the user both effort and money, and also putting less waste into the environment.